The present invention relates to correction coils and methods of operating corrections coils, particularly those employed in conjunction with superconductive NMR magnets for medical diagnostic imaging.
One of the stringent requirements for nuclear magnetic resonance (NMR) diagnostic medical imaging is for the establishment of a highly uniform magnetic field. This magnetic field may be provided by permanent magnets, resistive magnets, or superconductive magnets. The latter are particularly advantageous in that high strength fields may be created and maintained without large energy input requirements. However, in all of these NMR magnet structures, the intrinsic magnetic fields are generally not sufficiently homogenous to perform the desired imaging or spectroscopy measurements to desirable resolutions for medical imaging. To correct the magnetic field so as to improve its uniformity, several types of correction coils are employed and these are driven by a variety of current settings. In NMR imaging systems, cylindrical magnets are generally employed in order to provide a central bore into which the patient may be placed. Accordingly, it is often convenient to employ a cylindrical coordinate system for analysis, as is done herein. These systems generally employ a main magnet together with a number of correction coils. These correction coils include both axisymmetric coils and transverse correction coils. The axisymmetric coils are generally disposed in a helical pattern on a cylindrical coil form while the transverse correction coils are generally disposed in a so-called saddle shape disposed on a cylindrical surface. The present invention is directed to providing currents to the various correction coils.
An efficient means for correction current selection involves producing a magnetic field map for the volume of interest. In this volume, deviations from the mean are reduced by adding the magnetic field contributions of several correction coils as predicted by the well known method of least-squares polynomial approximation over a set of points. The problem, however, is that this method results in a Gaussian distribution over the set of points and this distribution is wasteful of the NMR magnet hardware in that a few extreme points on the tails of the Gaussian distribution broaden the error distribution by large amounts.
At the present time, NMR magnets are typically corrected by regression methods which employ an iterative process of measuring a field map and optimizing one correction coil contribution and then iteratively following through with measurement and correction cycles for the rest of the correction coils. This process requires a minimum number of measurement and correction cycles which is dictated by the number of orthogonal correction coil sets. However, this process could require several such passes through the correction coil set since this set cannot in general be guaranteed to be totally or perfectly orthogonal.
Alternatively and complementarily, Monte-Carlo methods have been employed. In this method correction coil currents are set randomly, one at a time, and the effects are observed. Then the measurement and correction and cycle is repeated. However, this process of determining correction coil currents is much more time consuming than necessary.